Alternative Definitions of the Terrestrial Reference System and Its Realization in Reference Frames

The International Terrestrial Reference System (ITRS) is defined by the IERS Conventions as a geocentric system with the origin in the Earth's centre of mass. It is realized by a crustfixed frame of reference stations (ITRF). The paper deals with alternative realizations of these specifications with the high accuracy needed in geosciences research. A geocentric frame fixes the origin permanently in the Earth's centre of mass, while a crust-fixed frame moves with the Earth's crust, and the origin of the coordinate system may depart from the geocentre ("geocentre motion"). The characteristics and realizations of both definitions are discussed along with their advantages and shortcomings. The computation of the reference frame is highly correlated with the observed network. In a global reference frame, the network stations should be distributed homogeneously over the Earth. Clusters of stations affect the frame by possible systematic (e.g. climatic) effects, in particular when applying similarity (Helmert) transformations. Densifications of the global frame in sparsely occupied regions of the network suffer from eventual distortions created by inhomogeneous station distributions. The time evolution of the reference frames is at present done by linear station coordinate changes (constant velocities) over long time intervals only. Seasonal variations are not considered. Experiences with the Chile 2010 earthquake demonstrate the necessity of successive reference frames with short time lag. Alternatives are discussed in the paper.